Purification of electrolytic copper

ABSTRACT

Apparatus for suspending and heating copper cathode plates in a hydrogen atmosphere for such purposes as oxygen removal, and in a vacuum for melting and removal of volatile or gaseous impurities by progressively melting with radiant heat the lower region of the suspended plates to create a thin molten film which drips off the plates.

United States Patent 72] Inventor Thomas Gordon Hart San Francisco,Calif. [21 Appl. No. 847,760 [22] Filed May 26, 1969 Division of Ser.No. 551,828, April 26, 1966, Pat. No. 3,501,292. [45] Patented Mar. 30,1971 [73] Assignee Phelps Dodge Corporation New York, N.Y.

[54] PURIFICATION OF ELECTROLYTIC COPPER Primary Examiner-Gerald A. DostAttorney-Davis, I-loxie, Faithfull & l-Iapgood ABSTRACT: Apparatus forsuspending and heating copper cathode plates in a hydrogen atmospherefor such purposes as 8 Claims 8 Drawing Figs oxygen removal, and in avacuum for melting and removal of [52] U.S. Cl 266/33, volatile orgaseous impurities by progressively melting with 1 75/65, 75/72 radiantheat the lower region of the suspended plates to create [51] Int. ClC2211 15/14 a thin molten film which drips off the plates.

i1 '1 I I I I /5 t S "I "1 '11 i "if I i I I 1 l3 1 '1 y I, i L IPatented March 30, 1971 3,572,670

3 Sheets-Sheet 1 X INVIiN'lUR. I 3 THOMAS 6.1mm

ATTORNEYS.

Patented March 30, 1971 3 Sheets-Sheet 2 I INVENTUR. THOMAS G. HART BYDwi, HM Fw W" A TTORNEYS.

Patented March 30, 1971 3 Sheets-Sheet 3 INVENIUR. THOMAS G. HART ATTORNEYS PURIFICATION OF ELECTROLYTIC COPPER This application is adivision of Ser. No. 551,828, filed Apr. 26, 1966, now US. Pat. No.3,501,292.

This invention relates to the purification of copper and particularly tothe purification of electrolytically refined copper by a processinvolving both hydrogen treatment and vacuum treatment.

The present invention is concerned with means for treating copper so asmainly to remove oxygen, sulfur and volatile impurities such as lead,bismuth and tellurium. Optimum purification is achieved according tothis-invention by turning to advantage certain peculiarities in theconfiguration, the structure and the impurity distribution ofelectrolytic cathode-plate copper so as to remove oxygen, sulfur andvolatile impurities from this copper in optimum fashion.

The impurity distribution of electrolytic cathode-plate copper,hereinafter called cathode copper, is peculiar in that the oxygen andsulfur impurities are largely confined to the surfaces. Further, cathodecopper being in the form of flat plates, affords an unusually largesurface area in relation to volume. Moreover, cathode copper has a finecrystalline structure which permits progressive'melting without tendencyto melt off in lumps as is the case with the coarse crystallinestructure of cast copper.

According to the invention, advantage is taken of the confinement to thesurfaces of the oxygen and sulfur impurities in cathode copper by theuse of surface treatments to remove these impurities. Surface treatmentof copper, particularly gaseous surface treatment, for removing oxygenand sulfur from the copper, has been discussed in my copending Ser. No.478,612, filed Aug. 10, 1965 and now abandoned. That applicationconcerns a novel method of combining hydrogen treatment with vacuumtreatment so as to remove oxygen, sulfur and volatile impurities fromany type of copper and makes particular note of the fact that oxygen andsulfur removal from cathode copper is most expeditiously accomplishedbefore the cathode copper is melted because the oxygen and sulfur'arethen most accessible and because the byproduct gases produced as aconsequence of the hydrogen action can then most easily escape. Thepresent invention supplements the teaching of Ser. No. 478,612 byteaching how, in addition to making use of the surface confinement ofimpurities of cathode copper, as by hydrogen treating prior to melting,effective use can also be made of the high surface area to volume ratioand the smooth melting characteristics of cathode copper for efficientlyfurther removing impurities therefrom.

The broad purpose of the present invention is accordingly to provide anoptimum method" for removing oxygen, sulfur and volatile impurities suchas lead, bismuth and tellurium from cathode copper. Somewhat narrowerpurposes are as follows:

First, to provide means for heating cathode copper in controlledatmosphere so as to substantially remove oxygen and sulfor, at leastfrom the surfaces;

second, to provide means for heating cathode copper, from which oxygenhas been substantially removed, in an atmosphere containing hydrogen soas to dissolve hydrogen in the copper;

third, to provide means for melting cathode copper in an atmospherecontaining hydrogen so as to dissolve hydrogen in the copper;

fourth, to provide optimum means for melting cathode copper in anatmosphere having very low pressure compared to atmospheric pressure soas to remove dissolved gasses and volatile impurities; and

fifth, to provide preferred apparatuses for accomplishing the abovepurposes.

Toward the above purposes, the following factors have particularimportance:

Insofar as is concerned the efficiency of hydrogen treatment of cathodecopper in removing sulfur and oxygen, an important factoris, asexplained at some length in my copending Ser. No. 478,612 that thistreatment be accomplished while the oxygen and sulfur are still largelyconfined to the surfaces which is to say before the cathode copper ismelted. Also of importance is that whereas the extent of oxygen removalfrom the solid cathode copper by prolonged hydrogen treatment is largelyindependent of the amount of sulfur present, the extent of sulfurremoval by this treatment may depend heavily on the amount of oxygenpresent. This is to say sulfur is not invariably removed from thesurfaces of solid cathode copper by prolonged heating in an atmospherecontaining hydrogen if oxygen is absent, depending absent, depending onsuch things as the nature of the compounds which contain the sulfur andupon the presence of constituents such as water vapor in the hydrogenbearing atmosphere, This is further to say that generally sulfur removalby hydrogen treatment is facilitated by the presence of an amount ofoxygen in the copper somewhat in excess of the amount normally found infresh cathode copper. Hence the purpose will be understood of slightlyfurther oxidizing the cathode copper surfaces, as by heating in air, forexample at 600 C. for five minutes, before hydrogen treatment.

Insofar as is concerned the efficiency of the hydrogen treatment ofsolid cathode copper in dissolving hydrogen in the copper, so as therebyto facilitate the-removal of volatile impurities by causing agitationduring the subsequent vacuum treatment as disclosed in Ser. No. 478,612,an important factor is that as much as possible of the solid coppersurface be freely exposed to the hydrogen, bearing atmosphere. This isbecause the rate at which hydrogen dissolves into the solid cathodecopper, at a particular temperature and with a particular proportion ofhydrogen in the atmosphere, is proportional to the copper surface areaexposed to the atmosphere. Hence the disadvantage will be understood, interms of length of time required, for example, tosaturate the solidcathode copper with hydrogen, of bundling the cathode copper together soas to be treated.

Insofar as is concerned the efficiency of the vacuum treatment of themolten cathode copper in removing dissolved gasses such as hydrogen andvolatile impurities such as lead, an important factor is again that amaximum copper surface area be exposed to the treatment. This is becausethe rates at which dissolved gasses and volatileimpurities are removedfrom molten copper, at a particular temperature and environmental gaspressure in a particular vacuum treatment apparatus, are proportional tothe molten copper surface area exposed to the vacuum treatment. Hence,the disadvantage will be understood, in terms of length of time requiredto, for example, substantially remove the lead from a batch of moltencopper, of containing this molten copper in, for example, a conventionalcrucible type melting vessel so as to be vacuum treated.

From the above it will be generally understood that the essence of thepresent invention is in taking advantage of the peculiar impuritydistribution, peculiar shape, and peculiar melting characteristics ofcathode copper so as to provide maximal surface exposure throughout thetreatment process and so as to-obtain thereby most efficient removal ofoxygen, sulfur and volatile impurities.

The means of the present invention for taking advantage of the peculiarcharacteristics of cathode copper as outlined above so as to accomplishefficient impurity removal will be understood form the followingdescriptions of preferred apparatus in conjunction with the followingdrawings of which:

FIG. 1 is a perspective view of a cathode copper plate;

FIG. 2 is a view partially in section of an apparatus for carrying outthe present invention;

FIG. 3 is a cross-sectional view of the apparatus of FIG. 2;

FIG. 4 is an enlarged view of the melting zone illustrated in FIG. 3;

Fig. 5 is a view partially in section of another apparatus embodying thepresent invention;

FIG. 6 is a view partially in section of another embodiment of thepresent invention;

FIG. 7 is an enlarged partial view in section showing a portion of theapparatus of FIG. 6; and

FIG. 8 is a perspective view of part of the apparatus of FIG. 6 showinghow the cathode plates are inserted into the apparatus of FIG. 6.

Attention is now directed to FIG. 1 which illustrates a typical cathodecopper plate hereafter termed a cathode." With reference to FIG. 1, thelength, the breadth, and the thickness of cathode 10 are respectivelyapproximately 3 feet, 3 feet, and three-fourths inch. Two identicalsuspension loops 11 are extensions of the thin copper starter" sheetonto which the cathode is electrolytically deposited. Suspension loops11 are primarily for suspending the starter sheet in the electrolytictank while the cathode is being deposited on the sheet, but, as will beseen later, are also used for suspending the cathode during purificationtreatment. The starter ,sheet, which does not appear as such in FIG. 1having become an integral part of the cathode, is usually deposited ontoon both sides and is therefore usually located approximately at thecenter of the cathode. It is of some importance in the manufacture ofcathodes intended for further purification that the starter sheets befree at least of oily contaminants before being used for deposition ofcathodes since these contaminants, becoming embedded within the cathode,can contribute undesirably to the gas evolution during the vacuumtreatment.

Attention is now directed to FIG. 2 which illustrates, somewhatdiagrammatically, the main features of an apparatus for progressivelymelting cathodes. With reference to FIG. 3 which is a cross-sectionalview of FIG. 2, 10 is the cross section of a cathode suspended by loop11 which is threaded by tube 12. 13 is one of a number of U-shapedheating elements spaced one behind the other along the breadth of thecathode, made for example of molybdenum wire, terminating at the upperends in terminals 14 and 15 across which a voltage is maintained so asto supply elements 13 with heating current. 16 is the cross section oftwo identical heat reflectors, located one each side of the heatingelements 13, extending along the breadth of the cathode, and moveable upand down the length of the cathode by means not shown. Heat reflectors16 are, for example, each made of a number of pieces of hightemperaturesteel sheet, these pieces being separated by refractory ceramic spacers17. 18 is the cross section of an enclosure such as a cylindrical steeltank, and 19 is the cross section of one of a number of water cooledcoils which girdle the tank so as to cool the walls. It will be readilyunderstood that in the absence of heat reflectors 16, for example ifreflectors 16 are moved down below the bottom of cathode 10, the loss ofheat by radiation from elements 13 and from cathode 10 heated byelements 13 is substantial. Hence, it will be understood that, withreflectors 16 below the bottom of cathode 10 and with elements 13supplied with sufficient electrical currentvia terminals 14 and 15 tomaintain cathode 10 at say about l,000 C., as reflectors 16 are moved upso as to envelop the bottom of cathode 10 the consequent reduction inheat loss from the bottoms of elements 13 and the' bottom of cathode 10is generally sufficient to raise the temperature of the bottoms ofelements 13 by a few hundred degrees and to melt the bottom of cathode10. Hence further, it will be understood that, with sufficient currentsupplied to elements 13 to raise the temperature of cathode 10 to sayabout l,000 C. in the absence of reflectors 16, as reflectors 16 areslowly moved from below the bottom of cathode 10 up to a position justbelow loops 11, cathode 10 will be progressively melted from bottom totop leaving unmelted only loops 1]. So that loops 11 retain sufficientstrength to support cathode 10 throughout the above melting operationloops 11 are cooled by circulating water through tube 12. The maximumspeed of movement of reflectors 16 so as to accomplish the progressivemelting of cathode 10, as above, is determined by a number of factorsincluding the type of atmosphere in enclosure 18, the width and spacingof elements 13, the spacing, size, number of layers and surfacecondition of reflectors 16, the surface condition of cathode 10 and thetemperature attained by cathode 10 in the absence of reflectors 16.However, any speed of movement of reflectors 16 below this maximum speedresults in uniform progressive melting from bottom to top of cathode 10,which is to say the length of cathode 10 is reduced at substantially thesame rate as the movement of reflectors 16, the melting bottom edge ofcathode 10 remaining within the gap between reflectors 16 and remainingsubstantially straight and parallel with the length of reflectors 16.Such unifonn progressive melting of cathode 10 from bottom to topwithout tendency to melt off irregularly in lumps leaving an irregularlower edge is, as already noted, largely due to the fine crystallinestructure of cathode 10 which is a normal characteristic ofelectrolytically deposited copper. The only impediment to uniformprogressive melting of cathode 10 as above would be the presence on thecathode of large nodules as can sometimes appear growing out from thesurfaces of cathode copper due to vagaries of the electrolyticdeposition process. If the nodules are large enough to significantlyaffect melting uniformity the inconvenience they cause in respect ofsuch matters as general handling and potential damage to heatingelements 13 in any case warrants their avoidance by closer control ofthe deposition process.

In continuation of the detail of FIG. 3, 20 is the drip channel whichcatches and leads away the molten copper dripping from cathode 10 ascathode 10 is progressively melted as above. Drip channel 20, made forexample of graphite, moves with reflectors 16, by means not shown, so asto maintain a fixed separation from the melting bottom edge of cathodel0 and is slightly sloped so that the molten copper flows towards oneend. The location of channel 20 within the U-shape of elements 13 andwithin the gap between reflectors 16--this region being tenned hereafterthe melt-regionensures that the temperature of channel 20 is maintainedabove copper melting temperature. Channel 20 empties into a holdingtrough 21, made for example of refractory ceramic, which is stationarywith respect to elements 13 and which is consequently separatedincreasingly from drip channel 20 as reflectors 16 and drip channel 20move up cathode 10. Partly to accommodate the increasing separation ofholding trough 21 from drip channel 20 so as to avoid splashing as wouldresult were channel 20 merely to drip at one end into trough 21,connection channel 22, made for example of graphite, connects at one endto the lower end of channel 20 and at the other end to one end of trough21; thus, as is better illustrated in side view in FIG. 2, one end ofchannel 22 moves with drip channel 20 while the other end of channel 22remains substantially stationary with holding trough 21. It will beunderstood that mere location within the U-shape of elements 13 may notbe sufficient to maintain channel 22 and trough 21 above the meltingpoint of copper when heating elements 13 are supplied with sufficientcurrent to melt cathode 10 in conjunction with the movement ofreflectors 16, as described above; accordingly, reflectors 23 which runparallel to and move with channel 22 and heat-insulating block 24 whichpartly surrounds trough 21 are provided to keep channel 22 and trough 21respectively above copper melting temperature.

For clearer understanding of some of the features described aboveattention is now directed to FIG. 2 which illustrates in side view theapparatus illustrated in cross-sectional view of FIG. 3. For clarity,reflectors 16 and 23 and insulating block ,24 are partially cut away toshow channels 20 and 22 and trough 21. Two cathodes 10 are shown side byside in FIG. 2 to illustrate how, by using an appropriate number ofheating elements 13 and appropriate lengths of parts such as channels,the apparatus is adapted to melting a number of cathodes simultaneously.End pieces to the apparatus, including the ends of enclosure 18, meansfor moving reflectors 16, means for controlling the atmosphere withinenclosure 18, means for placing cathodes within heating elements 13 andmeans for emptying trough 21, are not shown in FIG. 2. These end piecedetails are covered in a later description of a complete apparatus, asalso are other details incompletely shown in 2 and 3. The only detailsreferenced by number in 2 not referenced in 3 are pivot 25 and rocketarm 26, which are merely to indicate how, as explained above, one end ofconnecting channel 22 moves up with drip channel 20 as cathodes 10 aremelted while the other end stays substantially stationary with trough22.

From the above description of the main features of apparatus forprogressively melting cathodes in accordance with the present inventionit will be understood that in this apparatus progressive melting isaccomplished essentially by moving the melt region afforded byreflectors 16 and elements 13 from bottom to top of stationary cathodes.The movement between cathodes and melt region is of course relative,hence it will be readily understood that an alternative method ofprogressive melting is afforded by holding the melt region stationaryand by moving instead the cathodes so as to accomplish the same relativemovement as before. This alternative method of progressive melting maybe accomplished, for example, in apparatus broadly similar to theapparatus of FIGS. 2--t. In this connection attention is now directed toFIG. 5 which illustrates, again somewhat diagrammatically, the side viewof apparatus for progressive melting of cathodes by moving cathodesthrough a stationary melt region. With reference to FIG. 5, two cathodesare suspended side by side on rail 31 along which the cathodes are freeto move in horizontal direction within U-shaped heating elements 32 theheating elements 32 being similar in construction to the elements 13shown in FIG. 2. 33 is one of two stationary heat reflectors similar inshape and construction to movable reflectors 16 shown in FIG. 2. Theyare disposed one on each side of heating elements 32 so as to create inconjunction with the elements 32 a sloping melt region through whichcathodes 10 are moved by movement along rail 31. Within the U-shape ofelements 32 and the lower part of the gap between reflectors 33, runningparallel to reflectors 33 and hence within the melt re gion, is astationary channel 34, similar to channel 22 in FIG. 2, for collectingand leading away the molten copper dripping from cathodes 10 (reflector33 is partly broken away in two places to reveal channel 34). Belowchannel 34 and also within the U-shape of elements 32 is trough 35similar to trough 21 shown in FIG. 2 which runs parallel with thebottoms of elements 32. Partially enveloping trough 35 is heatinsulating block 36 (partly broken away in FIG. 5 to reveal trough 35).It will be noted that reflectors 33 extend only partially across the rowof heating elements 32, that is, there is a group of reflectorlesselements 32 which is not a part of the melt region. This group ofreflectorless elements is termed the preheat zone and serves to preheator maintain the temperature of the plates 10 as they are conveyed to thegroup of heating elements 32 which is termed the melt zone whichincludes the melt region. It will be further noted that the direction oftravel of cathodes 10 along rail 31 is from the preheat zone toward themelt zone and that, accordingly, one cathode I0 is shown in the preheatzone, and therefore shown unmelted at the bottom and is moving towardthe melt zone. The other cathode 10 in the melt zone is therefore shownmelted at the bottom with the bottom edge concealed within reflectors33. This cathode has traveled to its present position from the preheatzone at a rate sufficiently slow to ensure progressive melting aspreviously described.

It will be understood from the above that the arrangement of apparatusin FIG. 5 is such that cathodes may be continually fed into the preheatzone so as to continuously move along rail 31 through the preheat zoneand through the melt zone, and so as thereby to be continuouslypreheated and continuously progressively melted. Hence it will beunderstood that FIG. 5 illustrates a type of progressive meltingapparatus that is suitable for a continuous operation during which solidcathodes are continuously fed and molten copper is continuouslydischarged out of the apparatus. The advantage and disadvantage of thecontinuous operation of the apparatus of FIG. 5 as compared to the batchoperation of the apparatus of FIG. 2 will be brought out in laterdiscussion.

From the above descriptions of a batch version and a continuous versionof apparatus for progressively melting cathodes in accordance with thepresent invention it will by now be apparent that this apparatus isadaptable, by furnishing a controlled atmosphere around the cathodeswhile heating and melting, accomplishing at least part of the treatmentfor purifying cathode copper by the combined hydrogen treatment/vacuumtreatment mentioned earlier. This adaption will now be discussed inrelation to the batch apparatus of FIG.

Returning attention to FIG. 2 it will be evident that three distinctoperations may be conducted in this apparatus: Firstly, cathodes may bepreheated without melting, for an arbitrary length of time and to anarbitrary temperature below melting temperature, either by movingreflectors 16 below the bottom of the cathodes and by appropriatelyregulating the current through elements 13 or by moving reflectors 16 upand down at a fast enough rate to ensure melting does not occur and byappropriately regulating the current through elements 13; secondly, thecathodes may be progressively melted at an arbitrary rate below acertain maximum rate, as previously described; and thirdly, meltedcathodes may be retained molten in trough 22 for an arbitrary length oftime merely by continuing to supply current to elements I3 after themelting operation has been completed and by preventing trough 22 fromemptying. Furthermore it will be apparent, since there is no movement ofcathodes through the apparatus of FIG. 2, that once the cathodes are inposition within elements 13, the ends of enclosure 18 may be sealed offand a variety of atmospheres including, for example, ordinaryatmosphere, hydrogen bearing atmosphere and very low-pressureatmosphere, may be furnished within enclosure 18, assuming of course theability of parts of the apparatus such as elements 13 to withstand allthese atmospheres. Hence it will be understood that, for typicalexample, cathodes may be successively:

A. Heated in nonnal pressure air for 10 minutes at 600 C.;

B. Further heated in a normal pressure atmosphere composed of percentdry nitrogen and i5 percent dry hydrogen for 20 minutes at 1,000 C.;

C. Melted over a 10-minute period in a low-pressure atmosphere, thisatmosphere having a pressure equivalent to about 1 mm. of mercury andbeing largely composed of gases (mainly hydrogen) issuing from themelting and molten copper; and finally; and

D. Maintained molten for 10 minutes in a low-pressure atmosphere.

The above exampled operations A, B, C, and D constitute a typicaloptimum procedure (for a particular quality of cathode and a particulardesired level of impurity removal) for purifying cathode copper bycombined hydrogen treatment and vacuum treatment so as to remove oxygen,sulfur and volatile impurities. It remains therefore, in illustration ofthe advantages of the present invention, merely to point out how maximalsurface exposure, with advantages as previously outlined, is obtained bymeans of the apparatus of FIG. 2. In this connection the followingaspects of operations A, B, C, and D as related to the apparatus of FIG.2 may now be noted: Concerning operation A, which is the heating in airso as to slightly further oxidize the cathode surface for purpose offacilitating the sulfur removal, as previously described, this operationmay be conducted with reflectors 16 below the bottom of cathodes 10, sothat the only impediments to uniform circulation of air completelyaround cathodes 10 (this air being fed, for example, into one end ofenclosure 18 and discharged at the other) and hence uniform oxidation ofcathodes 10 are elements 13 which, however, being widely spaced relativeto thickness are of negligible encumbrance.

Concerning operation B, which is the heating in hydrogen bearingatmosphere so as to remove sulfur and oxygen and so as to dissolvehydrogen in the copper, much the same remarks apply as to operation Aabove which is to say almost ideal conditions pertain in regard touniform and unencumbered circulation of atmosphere completely aroundcathodes 10.

Concerning operation C, which is the melting in a low-pressureatmosphere so as to remove dissolved hydrogen and volatile impuritiesand which is conducted by progressively melting cathodes as previouslydescribed while constantly exhausting enclosure 18 by means of a vacuumpump (for example with both ends of enclosure 18 being completely sealedoff except for connection to the vacuum pump), this operation isconveniently assessed in two parts: Firstly, each portion of copper in acathode is exposed in thin molten section to the low-pressure atmosphereas each portion melts off the cathode. This is to say, the progressivemelting operation equivalently spreads a cathode out into a thin sheetof molten copper and exposes each portion of this molten sheet to thelow-pressure atmosphere. Secondly, each portion of a cathode afterprimary exposure to the low-pressure atmosphere while being melted thendrips into channel so as to obtain secondary exposure while flowing downchannels 20 and 22 into trough 21 and while accumulating in trough 21.This secondary exposure may also be visualized as to the equivalent ofthe exposure of cathode 10 spread out into the form of a thin sheet ofmolten copper.

The visualization of the primary and secondary exposure of a cathode inoperation C as equivalent to the exposure of a hypothetical thin sheetof molten copper, having the same volume but many times the surface areaof the cathode, to the to the low-pressure atmosphere for a hypotheticalperiod of the time depending on the melting rate, is useful in assessingthe relative importance of various factors affecting operation C. Forexample, if the melting bottom edge of cathode 10 is as illustrated inthe enlarged cross section of FIG. 4, which is to say melting occurs upthe sides of cathode 10 to a distance 11 of 2 inches, the primaryexposure of cathode 10 is roughly the equivalent of exposing a sheet ofmolten copper 270 square feet in area and 0.025 inch thick to thelow-pressure atmosphere for one-tenth second. lfdistance h is increasedto 4 inches (for the same total melting time of 10 minutes) thehypothetical sheet thickness is reduced to about 0.017 inch, the areabeing increased proportionately, and the hypothetical exposure time isincreased to about 0.15 second. As is well known in the vacuum treatmentart, increasing exposure area (which is, of course, the same asdecreasing sheet thickness for the same volume) and increasing exposuretime both contribute to increased impurity removal. Accordingly, theefficiency of the primary exposure of operation C in removing hydrogenand volatile impurities is at least in part affected by distance h, thegreater being distance h for the same total melting time the greater theefficiency. From this point of view there is therefore advantage inincreasing distance 11 by increasing the depth of the melt regionafforded by reflectors 16 in conjunction with elements 13, that is byincreasing the depth of reflectors 16. However, there is a limit to theusefulness of increasing the depth of reflectors 16 since an idealcondition for removing volatile impurities such as lead from cathode 10would be that reflectors 16 were not at all interposed between moltencopper flowing down the bottom of the cathode and the cooled walls ofenclosure 18 so that only the negligible encumbrance of elements 13impeded the direct ac cess of lead vapor to the cooled walls ofenclosure 18. Accordingly, increasing the depth of reflectors 16decreases the efficiency of removal of those impurities such as leadwhich condense on the cooled walls of enclosure 18, that is thoseimpurities termed volatile impurities. Hence, it will be understoodthat, depending in part upon the extent of volatile impurity removalrequired in a particular case, there will be an optimum depth forreflectors 16 which may typically range from l to 12 inches, dependingof course also on such other factors as the temperature of the cathodebefore the melting is performed and the distance between reflectors 16.Also signiticant in determining the efficiency of volatile impurityremoval in the primary exposure is the agitation of the molten copper onthe surface of the cathode (40 in F 16. 4) due to the rapid release ofhydrogen by the action of the low pressure. This agitation is manifestby frothing" of molten copper 40 as it flows down into drip 41 and dripsinto channel 20. Generally speaking, this frothing of molten copper 40equivalently decreases the aforementioned hypothetical sheet thicknessand increases the aforementioned hypothetical exposure time and henceincreases the efficiency of the primary exposure, particularly inremoving the volatile impurities. This improvement in efficiency due tothe frothing of molten copper 40 depends mainly on the amount ofdissolved hydrogen and on the rate of melting and may typically accountfor an increase in volatile impurity removal of two or three times ascompared to the amount removed under the same conditions but with nohydrogen present. It should be noted that the vigor of this frothingaction in part determines an optimum width for the U-shape of elements13 since small particles of molten copper can be ejected out from moltencopper 40 by vigorous frothing so as to escape the confines of elements13, and consequently channel 20, if the width of elements 13 is toosmall. Typically, for example, the loss of copper by frothing ejectionout of the sides is held to less than a tenth of a percent by an element13 width of 6 inches.

Concerning the secondary exposure of cathode 10, that is the exposurewhile dripping into channel 20, while flowing down channel 20 intochannel 22, while flowing down channel 22 into trough 21 and whilegathering in trough 21, it will be readily understood that thissecondary exposure adds considerably to the overall efficiency ofoperation C, even though of a somewhat different nature from the primaryexposure on the bottom of the melting cathode when the frothing actionis at its peak and when the molten copper surface is well defined. Thecontribution that this secondary exposure makes to the overallefficiency of operation C is, as already stated, also convenientlyassessed in the equivalent terms of hypothetical sheet thickness andhypothetical exposure time. Depending particularly on channel design, asis well known in the vacuum treatment art, this secondary exposure can,for example, increase the hypothetical equivalent exposure time anddecrease the hypothetical equivalent sheet thickness by as much as afactor of three in each case over the time and thickness equivalent tothe primary exposure alone. At this point it may conveniently be notedthat the reason for assessing the overall efficiency of operation C inequivalent terms of hypothetical sheet thickness and exposure timerather than, more simply, in terms of one figure of merit combining boththese factors is that the efficiency of gas removal and the efficiencyof volatile impurity removal depend somewhat differently on each ofthese two factors. This is to say, for example, that whereas theefiiciency of gas removal may be relatively unaffected by an increase inexposure time, the gas removal having been substantially completedbefore the exposure is completed, the efficiency of the volatileimpurity removal may be greatly affected, the volatile impurity removalnot having been completed when the exposure is complete. Hence, it willbe understood that in assessing the overall efficiency of operation C inthe equivalent terms of hypothetical sheet thickness and hypotheticalexposure time there is afforded a rough basis for determining,independently, the efiiciency of gas removal and the efiiciency ofvolatile impurity removal and for optimizing these two efficiencies inrelation to a particular requirement.

Concerning the final operation of the typical purification procedureconducted in the apparatus of FIG. 2 namely operation D, which is theholding of the molten copper in trough 21 under low-pressure atmospherefor 10 minutes, this operation has the least effect on the overallefficiency of the procedure. This is because by the time operation Dcommences, both the gas removal and the volatile impurity removal arenormally substantially completed by operation C. Accordingly, therelative inefficiency of operation D in removing gases and volatileimpurities, due to the relatively small area of exposure of moltencopper in trough 21, is of little consequence. The main purpose ofoperation D is hence not to contribute to the impurity removal, althoughin some measure it does, but rather to allow the temperature of moltencopper in trough 21 to be adjusted and to regularize preparatory tocasting, as will be explained later.

In conclusion of this discussion of a typical purification procedureconducted in the apparatus of FIG. 2, namely operations A, B, C, and D,it may be noted that B and C are the main operations and that theinclusion of operations A and D, in the form as described above, ismainly to show the relation of one or both of these optional operationsto the main operations when special problems are present. In practice,operation A, which may or may not be necessary depending on the cathodesurface condition as explained earlier, is more conveniently performedin separate apparatus since then apparatus of FIG. 2 may be constructedof materials which are ideally suited to operation in inert, reducing,or low-pressure atmospheres but which are not suited to operation in airat ordinary pressure, for example, molybdenum heating elements andgraphite channels. Also in practice, operation D may be modified byperforming in an atmosphere such as nitrogen at normal pressure ratherthan in low-pressure atmosphere, thus sacrificing impurity removal butobtaining the benefit of stabilizing temperature. Hence, it will beunderstood that the two operations which are of greatest significance inpurifying cathode copper according to the present invention are: (l)

prolonged heating of suspended cathodes below melting temperature whilefreely exposing the cathodes to an atmosphere containing hydrogenfollowed by (2) progressive melting of the suspended, heated cathodeswhile maintaining a low-pressure atmosphere. Further, it will by now beunderstood that these two operations are conducted with very highefficiency in apparatus such as diagrammatically illustrated in FIG. 2.

In order to give practical emphasis to the above, attention is nowdirected to FIG. 6 which illustrates a preferred practical apparatus forpurifying cathode copper in batches in accordance with the presentinvention. It will be noted by comparison with FIG. 2 that many ofthedetails of FIGS. 2 and 6 are substantially identical and consequentlysuch detail will not be further described.

With reference to FIG. 6, 50 is a cylindrical steel tank which enclosesthe apparatus and 60 is the inner frame which supports a progressivemelting assembly much as in FIG. 2. In

further detail of tank 50 (broken away to reveal frame 60), 51 v is ahinged outer door which in closed position makes a vacuum tight seal onouter flange 61; 62 is a hinged inner door having a slot 63 and inclosed position seats onto inner flange 64 so as to make a reasonablygas tight seal; 65 is one of two slides welded onto tank 50 for carryingframe 60 and facilitating the removal of frame 60 for maintenance; 66 isa flanged port for connection to vacuum manifold 67 and thence to thevacuum pumping system which is not shown; 68 is a flanged port having aremovable vacuum tight cover 69 for purpose to be explained later; 53and 54 are vacuum tight entries for rotatable shafts for purpose to beexplained later; 55 is a viewing port; and 56 is a utility port mainlyfor vacuum tight entry of water and electrical supplies.

In further detail of frame 60, 71 is one of the numerous U- shapedheating elements, 72 and 73 are respectively the upper and lowerreflectors, 74 and 75 are respectively the drip and connection channels,76 is the trough and 77 is the heat insulating block, all much aspreviously described in connection with FIG. 2; 78 is a cradle forcarrying reflector 72 and channel 74; shaft 79, counterweight 80,sprocket 81 and chain 82 are part of the conventional mechanicalarrangement for raising and lowering cradle 78 and thereby reflector 72and channel 74; to be explained in further detail later are 83, which isthe rail assembly for supporting elements 71 and cathodes for treatment,and 84 which is a shaft for operating a device for discharging trough76.

With reference now to FIG. 8 which illustrates in enlarged view the railassembly 83 and the means of support of a cathode on rail assembly 83,90 is one of two identical steel rails each being part of U-shaped steelchannels 91 which support copper terminal strips 92 which in turnsupport and supply electrical current to elements 71. Insulating strips93 are to prevent electrical contact between terminal strips 92 andchannels 91; water cooling tubes 94 and water cooling passage 95 are formoderately cooling assembly 83, terminal strips 92 and the loop portionsof the cathodes; and castor assemblies 97 and support bar 96 are forsupporting the cathodes along rails 90.

With reference now to FIG. 7 which illustrates the conventional devicefor discharging trough 76, 100 is a conventional graphite stopper rodwhich is raised and lowered via a linkage mechanism (not shown) operatedby shaft 84 which enters tank v50 via vacuum tight shaft entry 53. Thetapered hole in the bottom of trough 76 into which stopper rod 100 seatsis located above the center of port 68 in tank 50 so that, with cover 69removed and stopper rod 100 raised, molten copper in trough 76 flowsdirectly out of tank 50.

With reference now to FIGS. 6--8 a typical operation of the apparatuswill be described. From a cold start, that is with .all the suppliesturned off and the apparatus filled with air, the

first batch of cathodes is loaded with inner door 62 open, as shown inFIG. 6. Next both inner door 62 and outer door 51 are closed and thevacuum pumping system is turned on so as to exhaust the air from tank50. Next, the vacuum pumping system is turned off and a mixture ofnitrogen and hydrogen (for example percent hitrogen and 15 percenthydrogen) is.

allowed to fill tank 50, via a connection in utility port 56, and havingfilled tank 50 is allowed to flow through the tank at a moderate rate,exiting via a valve in vacuum manifold 67. Next, the electrical supplyto elements 71 and the water supply to the various water cooled partsare turned on and the current through elements 71 is regulated to bringthe temperature of the cathodes to between 800 C. and L000 C., thistemperature being measured by optical means through viewing port 55.Next, after about 20 minutes with the cathodes at between 800 C. andl,000 C., the flow of nitrogen-hydrogen mixture is cut off by closingboth inlet and outlet connections and the vacuum pumping system isturned on. Next, after the pressure in tank 50 has reduced to about 1mm. of mercury, as determined by a gauge connected to utility port 56,the upward movement of reflectors 72 is initiated, by switching on adrive motor connected to shaft 79, so as to initiate progressive meltingof the cathodes as previously described. Next, after completing theprogressive melting in a period of about 10 minutes, reflectors 72 arereturned to their lowermost position and the current through elements 71is regulated so as to bring the molten copper in trough 76 to thedesired casting temperature, which may be about l,l50 C., as determinedby thermocouples embedded in trough 76. Next, after the molten copper intrough 76 has attained the desired temperature, the vacuum pumpingsystem is switched off and nitrogen is admitted to tank 50, via a valvein manifold 67, and after filling tank 50 is then flowed through tank 50at slightly positive pressure, exiting via utility port 56. Next, cover69 on casting port 68 is removed and a copper casting mold which haspreviously been purged with nitrogen is connected to port 68. Next,stopper rod 100 is raised so as to allow molten copper to flow out oftrough 76 into the casting mold at an appropriate rate. Next, aftertrough 76 has emptied of molten copper and after the copperin thecasting mold has solidified throughout, the casting mold is removed fromport 68, cover 69 is replaced and the apparatus is now ready to treat afurther batch of cathodes. The purpose of inner door 63 will now beapparent since considerable time can be saved if the next batch ofcathodes can be loaded without necessity-of allowing the hot parts ofthe apparatus to cool: To load the next batch of cathodes outer door 51is opened while inner door 62 is kept closed, the caster assembliescarrying the loops from the previous batch are withdrawn and the newbatch of cathodes is loaded via slot 63 in door 62. Whilst thisunloading and loading operation is being accomplished the flow ofnitrogen into tank 50 is increased so as to prevent substantial entry ofair into tank 50 via slot 63, this being practical only because of therelatively small area of slot 63.-

From the above, at least the main details of the construction andoperation of an apparatus in accordance with the present invention forbatch purifying cathode copper while coincidentally melting and castingthis copper into useful shapes for fabrication of products, will by nowbe understood. It will be appreciated of course that minor details ofconstruction and operation have been omitted from the above description;these details are readily obtained from conventional practice. Itremains therefore in completion of this specification only to remarkupon the advantages and disadvantages of the abovedescribed batch typepurification apparatus as compared to a continuous type purificationapparatus, as may be based on FIG. 3.

A continuous purification apparatus is essentially one into whichunpurified material is continuously fed and out of which purifiedmaterial is continuously discharged. Accordingly, a continuous apparatusfor conducting a purification procedure such as described above is onein which cathodes are continuously passed into and preheated in hydrogenhearing atmosphere, are then continuously passed into and melted inlow-pressure atmosphere and are then continuously discharged out inpurified molten state. This is to say, visualizing, for example, such acontinuous apparatus based on FIG. 5, the preheat zone is separated fromthe melt zone by a vacuum lock through which pass hot solid cathodes andthe melt zone terminates in a second vacuum lock through whichdischarges purified molten copper. It will be readily understood thatsuch a continuous apparatus, particularly in respect of the vacuumlocks, is greatly more complicated and expensive than the batchapparatus of FIG. 6. However, it will equally be understood that thesimultaneous performance by this continuous apparatus of all parts ofthe treatment and the consequent elimination of purging cycles forreplacing one atmosphere with another and of loading and dischargingcycles, leads to a generally more efficient operation. Hence, finally,it will be understood that for a relatively small scale production andfor varying requirements as to degree of purification a batch apparatussuch as in FIG. 6 has considerable economic advantage which advantage,however, reduces in favour of a continuous apparatus such as may bebased on FIG. as the scale and consistency of production increases.

The apparatus of the present invention may of course be employed inrefining cathode copper where it is desired to melt the copper in ahydrogen-bearing atmosphere for example as is disclosed in my copendingapplication. Melting the plates in a hydrogen bearing atmosphere usingapparatus of the present invention affords an efficient means forintroducing large quantities of hydrogen into the molten copper sincethe primary exposure of a thin molten film of copper and the secondaryexposure of the molten copper as it drips and runs through the variouschannels of the apparatus of the present invention provide maximumexposure of the copper to the hydrogen atmosphere. Either the previouslydiscussed batch type or continuous type apparatus may of course be soemployed but the particular adaptability of the continuous typeapparatus illustrated in FIG. 5, will be well understood with referenceto the zone type" furnace described in that copending application.

I claim:

1. Apparatus for purifying copper, including an enclosure, suspensionmeans for suspending a copper cathode plate within said enclosure,heating means for heating and melting said suspended plate from bottomto top, and atmosphere controlling means for furnishing controlledatmosphere within said enclosure.

2. Apparatus as in claim 1, wherein said atmosphere controlling meansincludes means for generating a hydrogen-bearing atmosphere and meansfor generating an atmosphere having low pressure as compared toatmospheric pressure.

3. Apparatus for purifying copper, including an enclosure, suspensionmeans for suspending a copper cathode plate within said enclosure,heating means for heating the plate over an area of a face of the plate,said heating being sufiicient to melt copper over said area, means formoving the plate to cause said heating to occur over progressivelyhigher areas on said face to melt said plate from bottom to top, andatmosphere controlling means for fumrshmg con rolled atmosphere withinsaid enclosure.

4. Apparatus for purifying copper, including an enclosure, suspensionmeans for suspending a copper cathode plate within said enclosure,heating means for heating the plate over an area of a face of the plate,said heating being sufficient to melt copper over said area, means formoving the heating means to cause said heating to occur overprogressively higher areas on said face to melt said plate from bottomto top, and atmosphere controlling means for furnishing controlledatmosphere within said enclosure.

5. Apparatus as in claim 4, further including a channel below saidsuspended plate for transporting molten copper dripping from saidsuspended plate and a trough below said channel for accumulating moltencopper transported by said channel.

6. Apparatus for purifying copper including a sealed enclosure, meansfor suspending a cathode copper plate within the enclosure, means forcontrolling the atmosphere in the enclosure, heating means for heatingthe plate, heat-concentrating means movable with respect to the platefor raising the temperature of a portion of the face of the plate toabove melting temperature proceeding from bottom to top.

7. Apparatus as in claim 6, wherein said atmosphere controlling meansincludes means for furnishing an atmosphere at substantially atmosphericpressure containing hydrogen and means for furnishing an atmosphere atlow pressure compared to atmospheric pressure.

8. Apparatus as in claim 6, wherein said heating means includes aresistance heating element adjacent to said suspended plate and areflector adjacent to the heating element on the side of the heatingelement remote from the plate, outside said heating element, wherebyradiative heat loss from said heating element and from said suspendedplate is reduced.

2. Apparatus as in claim 1, wherein said atmosphere controlling meansincludes means for generating a hydrogen-bearing atmosphere and meansfor generating an atmosphere having low pressure as compared toatmospheric pressure.
 3. Apparatus for purifying copper, including anenclosure, suspension means for suspending a copper cathode plate withinsaid enclosure, heating means for heating the plate over an area of aface of the plate, said heating being sufficIent to melt copper oversaid area, means for moving the plate to cause said heating to occurover progressively higher areas on said face to melt said plate frombottom to top, and atmosphere controlling means for furnishingcontrolled atmosphere within said enclosure.
 4. Apparatus for purifyingcopper, including an enclosure, suspension means for suspending a coppercathode plate within said enclosure, heating means for heating the plateover an area of a face of the plate, said heating being sufficient tomelt copper over said area, means for moving the heating means to causesaid heating to occur over progressively higher areas on said face tomelt said plate from bottom to top, and atmosphere controlling means forfurnishing controlled atmosphere within said enclosure.
 5. Apparatus asin claim 4, further including a channel below said suspended plate fortransporting molten copper dripping from said suspended plate and atrough below said channel for accumulating molten copper transported bysaid channel.
 6. Apparatus for purifying copper including a sealedenclosure, means for suspending a cathode copper plate within theenclosure, means for controlling the atmosphere in the enclosure,heating means for heating the plate, heat-concentrating means movablewith respect to the plate for raising the temperature of a portion ofthe face of the plate to above melting temperature proceeding frombottom to top.
 7. Apparatus as in claim 6, wherein said atmospherecontrolling means includes means for furnishing an atmosphere atsubstantially atmospheric pressure containing hydrogen and means forfurnishing an atmosphere at low pressure compared to atmosphericpressure.
 8. Apparatus as in claim 6, wherein said heating meansincludes a resistance heating element adjacent to said suspended plateand a reflector adjacent to the heating element on the side of theheating element remote from the plate, outside said heating element,whereby radiative heat loss from said heating element and from saidsuspended plate is reduced.